CryptoWall, one of a family of malware programs that encrypts files and demands a ransom from victims, has undergone a revamp that is frustrating security researchers.

At one time, CryptoWall was a second-rate successor to CryptoLocker, which largely disappeared after law enforcement shut down the Gameover Zeus botnet that was used to distribute it.

Ransomware has been around for more than a decade, but cybercriminals have resurrected the scam over the last couple of years with surprising success. Files on computers infected with ransomware are encrypted, and victims are encouraged to pay a ransom -- usually in the virtual currency Bitcoin -- to unlock their files.

Dell SecureWorks estimated in August 2014 that CryptoWall had infected 600,000 computers in the previous six months, netting as much as $1 million in ransoms. The fee demanded ranges from $100 to $500.

CryptoWall uses strong public-key cryptography to scramble files with certain extensions. Aside from paying the ransom, the only other way to counter it is by restoring files from a backup, although CryptoWall hunts around and tries to encrypt those files as well.

Cisco's Talos Security Intelligence and Research Group has now analyzed a second version of CryptoWall that has improvements that make it harder to detect and study.

"It keeps evolving," said Earl Carter, a researcher with Cisco Talos, in a phone interview Thursday. Cybercriminals "seem to be continually morphing things, trying to make it more effective."

It is coded to run on both 32-bit and 64-bit systems, which increases its chances of running on whatever computer it infects, Carter said. Newer versions of Mac OS X and Windows are 64-bit operating systems.

The sample of CryptoWall analyzed by Cisco was sent via email in a ".zip" attachment. Contained in that attachment is an exploit that uses a Microsoft privilege escalation vulnerability, CVE-2013-3660, to gain greater control over the computer, Carter said.

If opened, CryptoWall doesn't decrypt its whole binary but instead just a small part, which then checks to see if it is running in a virtual environment, Carter said.

CryptoWall won't continue to decrypt itself if it is running in a virtual machine. Files are sometimes analyzed in a sandbox within a virtual machine to check if they're possibly malicious.

"They don't want people to easily look at this in a sandbox," Carter said.

A possible defense to CryptoWall is to add fake entries in the file system that indicate a virtual machine is running. Carter said that trick might work but is probably not a way to prevent such infections over the long term.

If CryptoWall decides it is safe to run after checking for a virtual machine, it continues to decrypt itself. It then communicates with command-and-control servers using the Tor network. Tor routes Internet traffic anonymously through a worldwide network of servers, making it harder to trace.

Researchers can't see the IP addresses of the servers that CryptoWall connects to, blocking further investigation of the servers used as part of its infrastructure, Carter said.